Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 151

8.5 Receptor-Like Kinases (RLKs): Can They Sense ABA
at the Cell Surface?
Cell surface-localized sensing mechanisms play fundamental roles in both plant
and animal cells, which perceive, through plasma membrane receptors, extracel-
lular signals from the environment and from other cells, activating downstream
signaling cascades. Receptor-like kinases (RLKs) are a class of plasma mem-
brane associated receptor kinase and found in metazoans and plants. Plant RLKs
comprise one of the largest families, with more than 610 and 1131 members in
Arabidopsis and rice ( Oryza sativa ), respectively (Shiu et al. 2004 ; Morillo and
Tax 2006 ). Structurally, a typical receptor kinase consists of an extracellular
domain, a single transmembrane region, and a cytoplasmic Ser/Thr protein kinase
domain. Extracellular domains vary greatly to perceive a wide range of signals or
stimuli, while intracellular kinase domains are relatively conserved to transduce
signals. The RLKs may be classified into multiple subfamilies on the basis of the
structure of their extracellular kinase domains. The biggest subfamily is leucine-
rich repeat RLKs (LRR-RLKs), and the other subfamilies include cysteine-rich
repeat (CRR) RLKs (CRKs), S-domain RLKs, domain of unknown function26
RLKs, and others (Shiu and Bleecker 2001 ). RLKs have been shown to regulate a
wide range of plant developmental processes and environmental responses, such as
the pathogen response, root, shoot and leaf development, meristem maintenance,
floral organ abscission, organ shape regulation, cellular differentiation, symbiosis,
self-incompatibility, and brassinosteroid signaling (reviewed in Morillo and Tax
2006 ; De Smet et al. 2009 ; Gish and Clark 2011 ).
An earlier study identified an ABA-induced LRR-RLK, RPK1, in Arabidopsis
(Hong et al. 1997 ), which contains an extracellular LRR and a Ser/Thr kinase
domain and are localized at the plasma membrane. Expression of the RPK1 gene
is also induced by dehydration, high salt, and low temperature (Hong et al.
1997 ), suggesting that RPK1 may be involved in plant response to environmental
stresses. Genetic approaches showed that the loss of function of RPK1 reduces
ABA sensitivity in seed germination, plant growth, stomatal closure, and gene
expression in Arabidopsis , revealing a positive role of RPK1 in ABA signaling
(Osakabe et al. 2005 ). Recently, RPK1 was shown to be involved in reactive oxy-
gen species (ROS) signaling during abiotic stresses, and overproduction of RPK1
enhances both water and oxidative stress tolerance in Arabidopsis (Osakabe et al.
2010 ). RPK1 was also reported to regulate ABA-induced leaf senescence (Lee
et al. 2011 ), suggesting that RPK1 have multiple functions in ABA and stress
signaling.
Hua et al. ( 2012 ) identified another member of the Arabidopsis LRR-RLK,
named GHR1, as a positive regulator of guard cell signaling in response to H 2 O 2
and ABA. The ghr1 mutation impaired H 2 O 2 activation of the calcium channel
and ABA/H 2 O 2 induction of stomatal closure. GHR1 functions downstream of the
cytosolic players ABI1 and ABI2 and may control stomatal movement by directly
interacting with, phosphorylating, and regulating SLAC1 anion channel, a key